In a TDD system two entities share a common carrier frequency channel for conducting bidirectional communications. In order to avoid interference and collisions, a first entity transmits during a first time period while a second entity receives the transmission from the first entity, and then the second entity transmits during a second time period while the first entity receives the transmission from the second entity. The first and second time periods are then repeated. In this manner it can be guaranteed that the two entities will not interfere with each others transmissions. Some amount of guard time can be inserted between the first and second periods, and between periods, to ensure that one entity does not begin transmitting before the other finishes.
At the start of a transmission the receiving entity must acquire the received signal, resolve any frequency uncertainty and lock to the carrier frequency and carrier phase, and then begin demodulating the modulated carrier. This can require some significant amount of time. Furthermore, the entire transmission might be missed if the receiver is unable to successfully acquire and lock to the received carrier during the transmission period.
As can be appreciated, it is desirable to provide an ability to make the received signal acquisition and carrier frequency lock processes occur as rapidly as possible. One approach provides a synchronous timing scheme wherein the basic frequency references of both entities are, ideally, identical. For example, the frequency reference of the AP or Hub is declared to be a master frequency reference, and the local frequency references of the CPEs are slaved to the AP frequency reference. Unfortunately, no frequency reference is ideal, and some drift can occur between the various frequency references of the CPEs and the AP. This frequency drift causes a phase and frequency shift between the AP and a receiving CPE, which can lead to received signal demodulation errors.